This invention relates to multiplexer circuits for switching signals and to electronic devices including such. More particularly, the invention is concerned with multiplexer circuits for switching signals from arrays of electrical elements or to arrays of electrical elements arranged in rows and columns and to column multiplexer circuits for use therewith. Particularly, but not exclusively, the electrical elements form the pixels of an image or fingerprint sensor array, and these pixels may include diodes.
Optical sensor arrays having pixels which include photodiodes are known, for example as described in WO97/25779. Fingerprint sensor arrays are also known which have pixels using diodes as switching elements, and having a capacitive element which stores a variable charge according to the profile of a fingerprint in contact with the array. In each case, enhanced spatial resolution is achieved by reducing the size of the pixels. However, external connection to the array becomes difficult as the pixel spacing is reduced.
It is known to use multiplexer circuits for external connection to rows or columns of pixels in high resolution arrays. Each multiplexer circuit links a number of inputs to a single output, which may then provide the external connections. External connections are not then required for each row or column, but may be provided only for the output of each multiplexer circuit. External conductors required for the control of the multiplexer circuits can be shared between all of the multiplexer circuits associated with the array, so that only a limited number of additional external connections are needed for control purposes. Thus, the total number of external connections needed for the array is greatly reduced by the use of multiplexer circuits.
Multiplexer circuits formed only from diodes are also known and referred to herein as diode multiplexer circuits. The use of diodes as the switching devices within the multiplexer circuits facilitates the integrated manufacture of a device which combines, on the same substrate, a diode-based pixel array and multiplexer circuits, both being fabricated by common processes.
An example of a diode multiplexer circuit used in conjunction with a device having an array of pixels is disclosed in WO 97/25779. This circuit has multiplexer switches connected to each column of the array. Each switch incorporates a diode bridge. The four connection points around the bridge define an input, output and two control terminals. The control terminals enable the four diodes to be switched between forward bias, when the input is linked to the output, and reverse bias, when the output is isolated from the input. Non-uniformity of the output characteristics of the diodes of the bridge gives rise to a DC offset current introduced between the voltage appearing on the output and a fixed voltage level. Some applications involve integrating a current signal at the column output, using a charge measurement circuit which operates at a fixed voltage. The DC offset must be integrated and subtracted out later in the signal handling chain, or be removed via an analogue offset removal circuit. The need to remove this offset current generally adds to the complexity of any system incorporating the multiplexer circuit.
Any reduction of the number of diodes needed for a multiplexer circuit will further facilitate the manufacture of these devices and may allow arrays incorporating them to achieve higher yields for a given array size.
According to the present invention, there is provided a multiplexer circuit for switching a selected one of a plurality of current inputs carried by respective input lines to a common output, the circuit comprising, for each input line:
a diode clamp comprising first and second clamp terminals and first and second clamp diodes arranged in series with the same polarity between the clamp terminals; and,
isolation means provided between each input line and the common terminal,
wherein the diode clamp is operable in two modes, a first mode in which voltages are applied to the clamp terminals such that the diodes of the diode clamp are forward biased and hold the input line at a first voltage which prevents the passage of current from the input line to the common output, and a second mode in which the voltages are applied to the clamp terminals such that the diodes of the diode clamp are reverse biased and the passage of the current from the input line to the common output is allowed.
Only two connections, for the diode clamp, are needed to control the switching of the current input. This is an improvement over the four connections needed for multiplexer circuits which incorporate a diode bridge in the switch.
The diode clamp used in the manner of the invention provides a simple diode-based circuit for controlling the passage of current from a number of inputs to a common output. In the second mode, the output current flows directly from the column, so no offset voltage is introduced by the clamp arrangement. Hence, the need to remove consequential offset currents added to the signal currents at a later stage is avoided.
The isolation means may comprise an isolation diode, and in this case the first voltage may be selected to reverse bias the isolation diode to prevent current flowing from the input line to the common output.
The isolation means may alternatively comprise an isolating capacitor, and in this case the first voltage may be selected depending on the source of the input current, such as to prevent current flowing from the source of the input current to the common output. The isolation capacitor does not itself prevent the passage of current, but it enables different input lines to be held at different voltages even though they are each coupled to a common output.
The present invention also provides an electronic device comprising an array of charge storage elements which are arranged in rows and columns and which are coupled to row and column conductors, the column conductors being arranged in at least one group, each group having a respective common output, a multiplexer circuit according to the present invention as set out above coupling the column conductors of the respective groups to the respective common output, and a charge measurement device which measures the flow of charge from the common output.
The charge storage elements may comprise photosensitive pixels including a photodiode and a switching diode. In this case, the first voltage may be selected to reverse bias the switching diode. Preferably, in this case, the isolation means comprises an isolation capacitor.
During the second mode, charge may be arranged to flow from each input to the respective isolation capacitor. The diode clamp may then be operable in a third mode in which voltages are applied to the clamp terminals such that the diodes in the diode clamp are forward biased and hold the input line at a second voltage which causes charge stored on the isolating capacitor to flow between the isolating capacitor and the charge measurement device.
This mode allows indirect reading of the signal charges from the charge storage elements by allowing charge to be transferred from the charge storage elements to the isolation capacitors when the diode clamp is in the second mode. This charge can subsequently be measured in the third mode by selectively draining the isolation capacitors through the respective diode clamps. This flow of charge can be measured by the charge measurement device.
The charge storage elements may comprise capacitive charge storage elements including two diodes and a variable capacitor, the current measurement being used to determine the capacitance. In this case, the isolation means may comprise an isolation diode.
As mentioned previously, the output signals of multiplexer circuits according to the present invention are free of the DC offset signal that is introduced by multiplexer circuits incorporating diode bridges in the switches. The present invention overcomes this problem by the current at the output being supplied directly by the current at the input rather than being supplied by an external source and only matched to the input current by a diode bridge.
The lack of DC offset eliminates the need for offset cancellation arrangements, such as subtracting out after integration by charge sensitive amplifiers or removal by an analogue offset removal circuit. Apart from simplifying the integrated circuit drivers, this allows for a faster read-out process since the need to allocate time slots for the operation of calibration loops or the like is removed. This speed may be traded off for a higher multiplexer ratio.
Multiplexer circuits according to the present invention require fewer diodes than multiplexer circuits employing diode bridges, allowing arrays including integrally formed multiplexers, fabricated together with the pixel array, to achieve better yields.
The elimination of the diode bridge reduces signal attenuation and means there is no direct feed of noise via the bridge to the amplifier. This reduces sensitivity to amplifier input offset voltages and improves the signal to noise ratio of the circuit.